Field of the Invention
The present invention relates to an organic light-emitting device (OLED), and more particularly, to an OLED which exhibits superior light extraction efficiency using an extraction structure for dipole light (light from dipole source) generated from an organic light-emitting layer.
Description of Related Art
In general, only about 20% of light generated from an OLED is emitted to the outside and about 80% of the light is lost by a waveguide effect originating from the difference in the refractive index between a glass substrate and an organic light-emitting layer which includes an anode, a hole injection layer, a hole transport layer, a light-emitting layer, an electron transport layer and an electron injection layer and by a total internal reflection originating from the difference in the refractive index between the glass substrate and the air. Specifically, the refractive index of the internal organic light-emitting layer 30 ranges from 1.7 to 1.8, whereas the refractive index of indium tin oxide (ITO) which is generally used for the anode is about 1.9. Since the two layers have a very small thickness ranging from 200 to 400 nm and the refractive index of glass used for the glass substrate is about 1.5, a planar waveguide is thereby formed inside the OLED. It is calculated that the ratio of the light lost in the internal waveguide mode due to the above-described reason is about 45%. In addition, since the refractive index of the glass substrate is about 1.5 and the refractive index of the ambient air is 1.0, when the light is directed outward from the inside of the glass substrate, a ray of light having an angle of incidence greater than a critical angle is totally reflected and is trapped inside the glass substrate. Since the ratio of the trapped light is up to about 35%, only about 20% of the generated light is emitted to the outside.
Dipoles are a type of light generated from the OLED, and the light extraction efficiency is significantly dependent on the direction in which dipoles oscillate. In general, dipoles oscillate in the direction vertical or parallel to a plane of the organic light-emitting layer. Dipoles oscillating in the vertical direction and dipoles oscillating in the parallel direction are uniformly distributed.
Among them, dipoles oscillating in the vertical direction have a significant contribution to the formation of the internal waveguide mode. That is, dipoles oscillating in the vertical direction are a cause that lowers the light extraction efficiency of the OLED.
In such an atmosphere where dipoles oscillating in the vertical direction continuously to exist, a significant improvement in the light extraction efficiency is not expectable even if, for example, a nanostructural internal light extraction layer having a light-scattering function is sandwiched between the anode and the glass substrate.
The information disclosed in the Background of the Invention section is provided only for better understanding of the background of the invention, and should not be taken as an acknowledgment or any form of suggestion that this information forms a prior art that would already be known to a person skilled in the art.